One branch of Islam (6)
'one branch of islam' is the definition.
(I've seen this in another clue)
This is all the clue.
(Another definition for shiism that I've seen is " Religion".)
'one branch of islam' is the definition.
(one of the main branches of Islam along with Sunni)
This is the entire clue.
(Other definitions for shia that I've seen before include "A_main branch of Islam" , "A major denomination of Islam" , "A main branch of Islam" , "One of the two main branches of orthodox Islam" .)